PROJECT SUMMARY/ABSTRACT The Ubiquitin Proteasome System (UPS) regulates the degradation of the majority of proteins in the cell and, as such, it is involved in essentially every cellular process. Because of its central role, misregulation within the UPS can potentiate or cause diseases, such as neurodegeneration and cancer. It is now well understood that protein misfolding and accumulation, which are intimately associated with neurodegenerative disease, can impair the UPS, exacerbating the disease. In fact, there is great interest to find ways of activating proteasome function as possible treatments for neurodegenerative disorders. To the contrary, in neoplastic disease the UPS is often exploited and even upregulated; due to this, a first line treatment for multiple myeloma is proteasome inhibition. The UPS thus sits at a shared and critical position in these two major human diseases. The proteasome?the central degradative machinery of the UPS?is regulated by very different regulatory complexes (e.g. 19S, PA28??, PA28?, PA200, and putatively P97). The job of these complexes is to regulate the function of the core particle of the proteasome, the 20S, which isolates its protein degradation chamber from the cellular milieu. A commonality shared by these regulators is that they all function to induce opening of the 20S proteasome substrate gate, which exposes substrates to the interior degradation chamber. The proteasome, and its regulators, provide a rich regulatory landscape to develop therapies that could profoundly impact these two large fields of study. This will require a deep biochemical understanding of the involved molecular mechanisms. The recent barrage of proteasomal structures facilitate this effort, but structures without an understanding of the dynamic mechanisms that underlie their functions are limited. Therefore, this proposal is primarily focused on understanding the biochemical function of three of these diverse proteasomal complexes and defining how they regulate protein degradation. We will focus on three specific questions: 1) How do the N-terminal domains of the proteasomal ATPases affect proteasome function?, 2) How does the mammalian P97 function to stimulate protein degradation by the 20S proteasome?, and 3) How does PA28? regulate 20S function to catalyze nuclear protein degradation? We have chosen to focus on these three regulators because they each play unique roles in the types of substrates that they degrade, and they each play key roles in specific human diseases. We implement a variety of approaches and systems to address these questions including studying function of proteasomal regulators from archaea, yeast, nematodes, mammals, and humans. Furthermore, we are using C. elegans as an animal model system to test our biochemically derived models and genetically test therapeutic concepts. The successful completion of this study will produce a sustained impact in the field by defining the central mechanisms of these three different cellular strategies for regulating protein degradation, each of which play different but critical roles in biology and disease.